1. Field of the Invention
The present invention relates to a road surface frictional coefficient estimating apparatus which estimates the frictional coefficient of a road surface on which a vehicle is traveling.
2. Description of the Related Art
As the technique for estimating the frictional coefficient (hereinafter referred to simply as “μ” in some cases) of a road surface on which a vehicle is traveling, the techniques disclosed in, for example, Patent Publication No. 3669668 (hereinafter referred to as “patent document 1”) and Japanese Patent Application Laid-Open No. 2003-118554 (hereinafter referred to as “patent document 2”), have been proposed by the present applicant.
According to the technique disclosed in patent document 1, a road surface reaction force acting on each wheel from a road surface (a cornering force (a lateral force of a vehicle) and a braking/driving force (a longitudinal force of the vehicle)) is estimated using a tire characteristic set on the basis of an estimated value of μ. Then, based on the estimated value of the road surface reaction force, the estimated value of a lateral acceleration of the vehicle and the estimated value of a yaw rate change velocity of the vehicle (the yaw rate change velocity at the center of gravity of the vehicle), which are motional state amounts of the vehicle and which occur due to the resultant force of the road surface reaction forces, are calculated. Further, according to the technique disclosed in patent document 1, a previous estimated value of μ is updated on the basis of the difference between the value of the lateral acceleration detected by an acceleration sensor and the estimated value of the lateral acceleration or the difference between the differential value of the yaw rate values detected by the yaw rate sensor (the detected value of the yaw rate change velocity) and the estimated value of the yaw rate change velocity, whichever difference is greater, thereby determining a new estimated value of μ.
According to the technique disclosed in patent document 2, a tire model set on the basis of the estimated value of μ is used to estimate the road surface reaction force acting on each wheel from a road surface (the cornering force and the braking/driving force). Then, based on the estimated value of the road surface reaction force, the estimated value of a lateral acceleration of the vehicle and the estimated value of the longitudinal acceleration of the vehicle indicative of the motional state amounts of the vehicle generated by the resultant force of the road surface reaction forces are calculated. According to the technique disclosed in patent document 2, in the case where a slip angle (side slip angle) of a rear wheel is small, the estimated value of μ is incremented or decremented by a predetermined value according to a magnitude relationship between the estimated value of the longitudinal acceleration of the vehicle and the detected value of the longitudinal acceleration provided by the sensor. In the case where the slip angle of a rear wheel is large, the estimated value of μ is incremented or decremented by a predetermined value according to the magnitude relationship between the estimated value of the lateral acceleration of the vehicle and the detected value of the lateral acceleration provided by the sensor. Thus, the estimated value of μ is sequentially updated.
The road surface reaction force acting on a wheel depends not only on μ but also on the slip rate or the side slip angle (slip angle) of a wheel. For this reason, according to the techniques disclosed in patent documents 1 and 2, the slip rate of a wheel is estimated and the side slip angle of a vehicle or the side slip angle of a wheel is also estimated using a motional model of the vehicle.
According to the techniques for updating the estimated value of μ on the basis of the difference between the estimated value of the lateral acceleration of the center of gravity of the vehicle and the value of lateral acceleration detected by the acceleration sensor (hereinafter referred to “the lateral acceleration difference”) or the difference between the estimated value of the yaw rate change velocity at the center of gravity of the vehicle and the detected value of the yaw rate change velocity based on an output of a yaw rate sensor (hereinafter referred to as “the yaw rate change velocity difference), as described in the aforesaid patent documents 1 and 2, it is assumed that the lateral acceleration difference or the yaw rate change velocity difference is caused by the error of an estimated value of μ used to determine the estimated value of the lateral acceleration or the estimated value of the yaw rate change velocity.
However, the lateral acceleration of the center of gravity of the vehicle and the yaw rate change velocity at the center of gravity are directly influenced by not only the value of μ but also the side slip motional state amounts (the temporal change rate of the side slip angle of the center of gravity of the vehicle and a side slip velocity). Further, the lateral acceleration of the center of gravity of the vehicle is also influenced by a bank angle of a road surface (the inclination angle about a roll shaft of the vehicle).
The value of the state amount of a side slip motion of a vehicle is generally difficult to accurately detect or estimate. The techniques described in patent documents 1 and 2 do not take the influence of a bank angle of a road surface into account.
Hence, the aforesaid lateral acceleration difference and the yaw rate change velocity difference are apt to be influenced by the estimation error of the state amount of the side slip motion of the vehicle or a bank angle in addition to an error of an estimated value of μ. This means that even if an estimated value of μ accurately coincides with an actual value, the lateral acceleration difference or the yaw rate change velocity difference easily fluctuates due to the influences of an estimation error of the state amount of the side slip motion of the vehicle or a bank angle. As a result, even if the estimated value of μ is updated according to the lateral acceleration difference or the yaw rate change velocity difference, the error of an estimated value of μ will not be properly reflected, making it difficult to accurately and stably determine the estimated value of μ in some cases.